Optical scanner, image forming device, optical scanning method

ABSTRACT

A technique capable of achieving reduction in space of allocating an optical system and improvement of an optical characteristic of a scan light in an optical scanner that scans a light flux from a light source on each of a photosensitive surface of a plurality of photoconductors in a main-scanning direction is provided. 
     An optical scanner comprising: a polygon mirror  80 ; a pre-deflection optical system  7 ; and a post-deflection optical system A, wherein the post-deflection optical system A includes a common optical element having a smooth surface acting on all the light fluxes reflected and deflected by each of the plurality of reflecting surfaces in the polygon mirror  80 , the common optical element that applies power to the light flux reflected and deflected by the polygon mirror  80  and introduced to each of the plurality of photoconductors, so as to make the light flux introduced to the photosensitive surface by the post-deflection optical system A to have a predetermined optical characteristic on the photosensitive surface depending on an incident position of the light flux.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical scanner that scans a lightflux from a light source to a photosensitive surface of each of aplurality of photoconductors in a main-scanning direction. Inparticular, the present invention relates to a technique of achievingreduction in space for allocation of an optical system and improvementof an optical characteristic of scan light.

2. Description of the Related Art

Conventionally, in relation to an optical scanning technique that scansa light flux from a light source to a photosensitive surface of each ofa plurality of photoconductors in a main-scanning direction, there hasbeen disclosed an image forming device that forms an image such as anelectrostatic latent image on the photoconductors by irradiation of alight beam. The image forming device sets a plurality of reflectingsurfaces in a rotational deflector that scans the light beam to theplurality of photoconductors to have an inclination angle different fromone another with respect to a rotational axis, and scans the differentphotoconductor for each of the reflecting surfaces having theinclination angle different from one another (for example, refer toPatent Document 1: Jpn. Pat. Appln. Publication No. 2000-2846 and PatentDocument 2: Jpn. Pat. Appln. Publication No. 11-218991).

In the conventional art configured as described above, a polygon mirrorhaving the reflecting surfaces each having a reflecting angle differentfrom one another is used, so that both kinds of scanning, that is,scanning by and switchover of a light path of a laser beam, can becarried out by a rotational operation of a polygon mirror. Therefore,lowering of cost by reduction in the number of items of parts and byreduction in movable parts, and printing with high precision bysimplification of control operation can be achieved.

In the conventional art described above, there also is an invention inwhich a cylinder lens is arranged in a pre-deflection optical system(refer to Patent Document 1). In this invention, light is considered tobe converged in the vicinity of a reflecting surface of the deflector ina sub-scanning direction, and in the post-deflection optical system, thereflecting surface and the image surface of the deflector are consideredto be made to have a substantially conjugate relationship in thesub-scanning direction so as to provide a surface tilt correctingfunction. On the other hand, there is a configuration where the cylinderlens is not included in the pre-deflection optical system. In such aconfiguration, the post-deflection optical system is considered not tohave the surface tilt correcting function (for example, refer to PatentDocument 2 and Patent Document 3: Jpn. Pat. Appln. Publication No.63-273814).

Conventionally, in a case where each of the reflecting surfaces of thepolygon mirror is made to have an angle different from one another and alight beam is imaged onto each exposure target part by thepost-deflection optical system that has the surface tilt correctionfunction, each of the light beams deflected on each of the surfaces ofthe polygon mirror has been imaged in an optical part different from oneanother. In this case, as much as there is a need to secure certainspace between the optical parts, there is a need to make the inclinationangle applied to each of the reflecting surfaces of the polygon mirrorlarge, and the space for allocating the optical system in thesub-scanning direction is enlarged.

In addition, by securing the certain space between the optical parts,generation of an asymmetric wave aberration which becomes larger whenthe inclination angle applied to each of the reflecting surfaces of thepolygon mirror is large becomes difficult to restrict. This has led todeterioration in an imaging characteristic.

Further, in a case where the post-deflection optical system does nothave the surface tilt correcting function, an allowable amount ofsurface tilt of each of the reflecting surfaces of the polygon mirrorbecomes extremely small, and there is a problem that manufacturing costof the polygon mirror significantly increases. In addition, in a casewhere the lens does not have the surface tilt correcting function andthe configuration is such that light beam passes through the common lensfrom each of the reflecting surfaces of the polygon mirror to which anangle is applied, a scan line is curved and overlapping of colors withhigh precision (registration) cannot be carried out. In particular, in acase where a full color image is formed by using four sets of beams inorder to form four latent images, an image has been significantlydeteriorated due to color shift.

SUMMARY OF THE INVENTION

An object of an embodiment of the present invention is to provide atechnique capable of achieving reduction in space for allocating anoptical system and improvement in an optical characteristic of scanlight in an optical scanner that scans a light flux from a light sourceon a photosensitive surface of each of a plurality of photoconductors ina main-scanning direction.

In order to achieve the above object, according to an aspect of thepresent invention, there is provided an optical scanner that can scan alight flux from a light source to a photosensitive surface of each of aplurality of photoconductor in a main-scanning direction, comprising: arotational deflector that reflects and deflects an incident light fluxby a plurality of reflecting surfaces arranged corresponding to theplurality of photoconductors in a rotational direction to scan theincident light flux in the main-scanning direction, the rotationaldeflector in which an inclination angle of the plurality of reflectingsurfaces with respect to a rotational axis of the rotational deflectoris set to an angle depending on the photoconductor to which each of thereflecting surfaces corresponds; a pre-deflection optical system thatshapes the light from the light source into a light flux having apredetermined cross-sectional shape and introduces the light flux to therotational deflector, and also converges the light flux in asub-scanning direction in the vicinity of the reflecting surfaces of therotational deflector; and a post-deflection optical system thatintroduces the light flux reflected and deflected by each of theplurality of reflecting surfaces in the rotational deflector to thephotosensitive surface of the photoconductor corresponding to each ofthe reflecting surfaces, wherein the post-deflection optical systemincludes a common (commonly-used) optical element that applies power toa light flux reflected and deflected in the rotational deflector andintroduced to each of the plurality of photoconductors, so as to makethe light flux introduced to the photosensitive surface by thepost-deflection optical system to have a predetermined opticalcharacteristic on the photosensitive surface depending on an incidentposition of the light flux.

In addition, according to an aspect of the present invention, there isprovided an image forming device comprising: the optical scannerconfigured as described above; a photoconductor that has anelectrostatic latent image formed thereon by a light flux scanned by theoptical scanner; and a developing unit that develops the electrostaticlatent image formed on the photoconductor.

In addition, according to an aspect of the present invention, there isprovided an optical scanner that can scan a light flux from a lightsource to a photosensitive surface of each of a plurality ofphotoconductors in a main-scanning direction comprising: a light fluxdeflecting unit that reflects and deflects an incident light flux by aplurality of reflecting surfaces arranged corresponding to the pluralityof photoconductors in a rotational direction to scan the incident lightflux in the main-scanning direction, the light flux deflecting unit inwhich an inclination angle of the plurality of reflecting surfaces withrespect to a rotational axis of the light flux deflecting unit is set toan angle depending on the photoconductor to which each of the reflectingsurfaces corresponds; a pre-deflection light guiding unit that shapesthe light from the light source into a light flux having a predeterminedcross-sectional shape and introduces the light flux to the light fluxdeflecting unit, and also converges the light flux in a sub-scanningdirection in the vicinity of the reflecting surfaces of the light fluxdeflecting unit; and a post-deflection light guiding unit thatintroduces the light flux reflected and deflected by each of theplurality of reflecting surfaces in the light flux deflecting unit tothe photosensitive surface of the photoconductor corresponding to eachof the reflecting surfaces, wherein the post-deflection light guidingunit includes a common (commonly-used) optical element that appliespower to a light flux reflected and deflected in the light fluxdeflecting unit and introduced to each of the plurality ofphotoconductors, so as to make the light flux introduced to thephotosensitive surface by the post-deflection light guiding unit to havea predetermined optical characteristic on the photosensitive surfacedepending on an incident position of the light flux.

Further, according to an aspect of the present invention, there isprovided an optical scanning method that scans a light flux from a lightsource to a photosensitive surface of each of a plurality ofphotoconductors in a main-scanning direction, comprising: shaping thelight from the light source into a light flux having a predeterminedcross-sectional shape to introduce the light flux to the rotationaldeflector, and also converging the light flux in a sub-scanningdirection in the vicinity of the reflecting surfaces of the rotationaldeflector; reflecting and deflecting an incident light flux by means ofthe rotational deflector which has a plurality of reflecting surfacesarranged corresponding to the plurality of photoconductors in arotational direction, and in which an inclination angle of the pluralityof reflecting surfaces with respect to a rotational axis of therotational deflector is set to an angle depending on the photoconductorto which each of the reflecting surfaces corresponds, to scan theincident light flux in the main-scanning direction; and applying powerto the light flux reflected and deflected by the rotational deflectorand introduced to each of the plurality of photoconductors by means of acommon (commonly-used) optical element so as to make the light fluxintroduced to the photosensitive surface to have a predetermined opticalcharacteristic on the photosensitive surface depending on an incidentposition of the light flux, and introducing the light flux to which thepower is applied by the common (commonly-used) optical element to thephotosensitive surface of the photoconductor corresponding to each ofthe reflecting surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a configuration of an optical system of anoptical scanner according to a first embodiment of the present inventionin a state where turning back by a mirror is extended;

FIG. 2 is a view showing a schematic configuration of an image formingdevice including the optical scanner according to the first embodiment;

FIG. 3 is a view showing an optical path in the optical system in theoptical scanner according to the first embodiment;

FIG. 4 is a view showing an optical path of a light flux guided to aplurality of photoconductors in the optical scanner according to thefirst embodiment in an enlarged manner in a sub-scanning direction in astate where the turning back by the mirror is extended;

FIG. 5 is an enlarged view of the vicinity of a common (commonly-used)fθ lens in FIG. 4;

FIG. 6 is a view showing an example of a power distribution of an fθ1lens 111 in the sub-scanning direction;

FIG. 7 is a view showing an example of a power distribution of an fθ2lens 112 in the sub-scanning direction;

FIG. 8 is a view showing a relationship between space between principalrays of light fluxes of light sources positioned on both ends among aplurality of light sources in the sub-scanning direction, and a positionin an optical axis direction;

FIG. 9 is a view showing a relationship between the space of theprincipal rays of the light fluxes from the light sources positioned onboth ends among the plurality of light sources in the sub-scanningdirection, and the position in the optical axis direction;

FIG. 10 is a view showing a relationship between the space of theprincipal rays of the light fluxes from the light sources positioned onboth ends among the plurality of light sources in the sub-scanningdirection, and the position in the optical axis direction;

FIG. 11 is a view showing a relationship between the space of theprincipal rays of the light fluxes from the light sources positioned onboth ends among the plurality of light sources in the sub-scanningdirection, and the position in the optical axis direction;

FIG. 12 is a view showing an example of allocation of an optical systemof a conventional optical scanner included in a color image formingdevice;

FIG. 13 is a view showing the optical path in the optical system in theoptical scanner according to a second embodiment of the presentinvention;

FIG. 14 is a view showing the optical path of the light flux guided tothe plurality of photoconductors in the optical scanner according to thesecond embodiment in an enlarged manner in a sub-scanning direction in astate where turning back by the mirror is extended;

FIG. 15 is an enlarged view of the vicinity of a common (commonly-used)fθ lens in FIG. 14;

FIG. 16 is a view showing the relationship between the space of theprincipal rays of the light fluxes from the light sources positioned onboth ends among the plurality of light sources in the sub-scanningdirection, and the position in the optical axis direction according tothe second embodiment;

FIG. 17 is a view showing the relationship between the space of theprincipal rays of the light fluxes from the light sources positioned onboth ends among the plurality of light sources in the sub-scanningdirection, and the position in the optical axis direction according tothe second embodiment;

FIG. 18 is a view showing the relationship between the space of theprincipal rays of the light fluxes from the light sources positioned onboth ends among the plurality of light sources in the sub-scanningdirection, and the position in the optical axis direction according tothe second embodiment;

FIG. 19 is a view showing the relationship between the space of theprincipal rays of the light fluxes from the light sources positioned onboth ends among the plurality of light sources in the sub-scanningdirection, and the position in the optical axis direction according tothe second embodiment;

FIG. 20 is a view showing a power distribution of an fθ lens 110′ in thesub-scanning direction according to the second embodiment;

FIG. 21 is a view showing a position of a light beam incident on anincident surface of a cylinder lens 120 y in the sub-scanning directionaccording to the second embodiment;

FIG. 22 is a plan view showing a configuration of the optical system inthe optical scanner according to a third embodiment of the presentinvention in a state where turning back by the mirror is extended;

FIG. 23 is a view showing a schematic configuration of an image formingdevice 900 including the optical scanner according to the thirdembodiment;

FIG. 24 is a view showing the optical path in the optical system in theoptical scanner according to the third embodiment in a state whereturning back by the mirror is extended;

FIG. 25 is a view showing the optical path of the light flux guided tothe plurality of photoconductors in the optical scanner according to thethird embodiment in an enlarged manner in a sub-scanning direction in astate where turning back by the mirror is extended;

FIG. 26 is an enlarged view of the vicinity of the common(commonly-used) fθ lens in FIG. 25;

FIG. 27 is a view showing the relationship between the space (verticalaxis) of the principal rays of the light fluxes from the light sourcespositioned on both ends among the plurality of light sources in thesub-scanning direction, and the position (horizontal axis) in theoptical axis direction according to the third embodiment;

FIG. 28 is a view showing the relationship between the space (verticalaxis) of the principal rays of the light fluxes from the light sourcespositioned on both ends among the plurality of light sources in thesub-scanning direction, and the position (horizontal axis) in theoptical axis direction according to the third embodiment;

FIG. 29 is a view showing the relationship between the space (verticalaxis) of the principal rays of the light fluxes from the light sourcespositioned on both ends among the plurality of light sources in thesub-scanning direction, and the position (horizontal axis) in theoptical axis direction according to the third embodiment;

FIG. 30 is a view showing the relationship between the space (verticalaxis) of the principal rays of the light fluxes from the light sourcespositioned on both ends among the plurality of light sources in thesub-scanning direction, and the position (horizontal axis) in theoptical axis direction according to the third embodiment;

FIG. 31 is a view showing a power distribution of an fθ lens 110′ in thesub-scanning direction according to the third embodiment;

FIG. 32 is a view showing the optical path in the optical system in theoptical scanner according to a fourth embodiment of the presentinvention in a state where turning back by the mirror is extended;

FIG. 33 is a view showing the optical path of the light flux guided tothe plurality of photoconductors in the optical scanner according to thefourth embodiment in an enlarged manner in a sub-scanning direction in astate where turning back by the turning back mirror is extended;

FIG. 34 is an enlarged view of the vicinity of the common(commonly-used) fθ lens in FIG. 33;

FIG. 35 is a view showing the relationship between the space (verticalaxis) of the principal rays of the light fluxes from the light sourcespositioned on both ends among the plurality of light sources in thesub-scanning direction, and the position (horizontal axis) in theoptical axis direction according to the fourth embodiment;

FIG. 36 is a view showing the relationship between the space (verticalaxis) of the principal rays of the light fluxes from the light sourcespositioned on both ends among the plurality of light sources in thesub-scanning direction, and the position (horizontal axis) in theoptical axis direction according to the fourth embodiment;

FIG. 37 is a view showing the relationship between the space (verticalaxis) of the principal rays of the light fluxes from the light sourcespositioned on both ends among the plurality of light sources in thesub-scanning direction, and the position (horizontal axis) in theoptical axis direction according to the fourth embodiment;

FIG. 38 is a view showing the relationship between the space (verticalaxis) of the principal rays of the light fluxes from the light sourcespositioned on both ends among the plurality of light sources in thesub-scanning direction, and the position (horizontal axis) in theoptical axis direction according to the fourth embodiment;

FIG. 39 is a view showing a power distribution of the fθ1 lens 111 inthe sub-scanning direction according to the fourth embodiment;

FIG. 40 is a view showing a power distribution of the fθ2 lens 112 inthe sub-scanning direction according to the fourth embodiment;

FIG. 41 is a view showing a result of examination on which lens surfaceof the fθ1 lens and the fθ2 lens a free-form surface having power actingon all the light fluxes guided to the plurality of photoconductors needsto be formed;

FIG. 42 is a view showing an example in which a diffractive opticalelement surface (indicated by a bold line) is formed on an exit surfaceside of the fθ2 lens 112 in the configuration shown in FIG. 3;

FIG. 43 is a view showing an example in which the diffractive opticalelement surface (indicated by a bold line) is formed on an exit surfaceside of the fθ2 lens 112 in the configuration shown in FIG. 32;

FIG. 44 is a view showing an example in which the diffractive opticalelement surface (indicated by a bold line) is formed on an exit surfaceside of the fθ lens 110′ in the configuration shown in FIG. 13; and

FIG. 45 is a view showing an example in which the diffractive opticalelement surface (indicated by a bold line) is formed on an exit surfaceside of the fθ lens 110′ in the configuration shown in FIG. 24.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings.

First Embodiment

First, a first embodiment of the present invention will be described.FIG. 1 is a plan view showing a configuration of an optical system in anoptical scanner according to the present embodiment in a state whereturning back of a turning back mirror is extended. FIG. 2 is across-sectional view in a sub-scanning direction showing a schematicconfiguration of an image forming device 900 including the opticalscanner according to the present embodiment. FIG. 3 is a view showing anoptical path in the optical system in the optical scanner according tothe present embodiment. FIG. 4 and FIG. 5 are cross-sectional views inthe sub-scanning direction showing the optical path of an optical fluxguided to a plurality of photoconductors in the optical scanneraccording to the present embodiment in a state where the turning back bythe mirror is extended.

As shown in FIGS. 1 and 2, an optical scanner 1 according to the presentembodiment includes a pre-deflection optical system (pre-deflectionlight guiding unit) 7, a polygon mirror (rotational deflector, lightflux deflecting unit) 80, and a post-deflection optical system(post-deflection light guiding unit) A.

The optical scanner 1 has a role of scanning the light flux from a lightsource to a photosensitive surface of each of a plurality ofphotoconductors 401 y to 401 k in a main-scanning direction. Anelectrostatic latent image is formed on each of the photosensitivesurfaces of the photoconductors 401 y to 401 k by the light flux scannedby the optical scanner 1. The electrostatic latent image formed on eachof the photoconductors is developed by a developer of a colorcorresponding to each of the photoconductors by developing units 501 yto 501 k.

Hereinafter, a detail of the optical scanner 1 according to the presentembodiment will be described.

The polygon mirror 80 reflects and deflects an incident light flux by aplurality of reflecting surfaces 80 y to 80 k arranged corresponding toeach of the plurality of photoconductors 401 y to 401 k in a rotationaldirection, thereby scanning the incident light flux in the main-scanningdirection. In addition, an inclination angle of each of the plurality ofreflecting surfaces 80 y to 80 k of the polygon mirror 80 with respectto a rotational axis of the polygon mirror 80 is set to an angledepending on the photoconductor to which each of the reflecting surfacescorresponds. In a configuration such as above, the number of thereflecting surfaces of the polygon mirror 80 is a multiple of the numberof colors. Here, in order to use four colors, yellow (401 y), magenta(401 m), cyan (401 c), and black (401 k), the number of the reflectingsurfaces of the polygon mirror 80 is a multiple of four (4, 8, 12, . . .).

The pre-deflection optical system 7 includes a light source 71 includingan LD array having four light sources arranged in positions differentfrom one another in the sub-scanning direction (a direction along therotational axis of the polygon mirror) crossing the main-scanningdirection at right angle, each of the four light sources capable ofblinking independently, a finite lens (or a collimator lens) 72 thatconverts divergent light from the light source 71 into convergent light,parallel light, or reduced divergent light, an aperture 73, and acylinder lens 74 that converges the light flux in the vicinity of thepolygon mirror 80.

The pre-deflection optical system 7 has a configuration as describedabove, and thereby shapes light from the light source 71 into the lightflux, for example, with a predetermined cross-sectional shape having alonger diameter in the main-scanning direction, to guide the light fluxtoward the polygon mirror 80, and also focuses the light flux in thesub-scanning direction in the vicinity of the reflecting surfaces of thepolygon mirror 80.

The post-deflection optical system A includes an fθ1 lens 111 and an fθ2lens 112 made of a resin material such as plastic and having a free-formsurface of a power distribution in which power changes continuously,cylinder lenses 120 y to 120 k (a plurality of optical elements)provided on an optical path between a common (commonly-used) opticalelement and each of the photoconductors corresponding to each of thephotoconductors 401 y to 401 k and having a positive power with a convexsurface on an incident surface side, and a cover glass 130 used forpreventing dirt and dust from entering into the optical scanner.

By a configuration as described above, the post-deflection opticalsystem A guides light fluxes Ly to Lk reflected and deflected by each ofthe plurality of reflecting surfaces 80 y to 80 k of the polygon mirror80 to the photosensitive surfaces of the photoconductors 401 y to 401 kcorresponding to each of the reflecting surfaces in the optical pathdifferent from one another. In the present embodiment, there are eightreflecting surfaces on the polygon mirror 80. Therefore, when there isone light flux incident on the polygon mirror, one revolution of thepolygon mirror 80 can write color information of four colors on each ofthe photoconductors twice. Here, the light source 71 adopts a so-called“multi-beam optical system” that exits four light fluxes, each of whichindependently forms an electrostatic latent image on the photosensitivesurface. Therefore, one revolution of the polygon mirror 80 can writeeight lines (four lines×2) of color information of four colors on eachof the photoconductors once.

In addition, the light sources integrated in one array are also used forimage forming processing of primary colors (black, cyan, magenta, andyellow) corresponding to each of the photoconductors, and thereby thenumber of optical parts can be reduced to attempt cost reduction andalso reduction in allocation space can be achieved. In a case only onelight source is allocated to be used for all the photoconductors, inorder to form a latent image of four colors, the number of revolutionsof the polygon mirror and a driving frequency of an LD need to be fourtimes as many as the case where one light source is provided for each ofthe photoconductors. For this reason, the image forming processing inhigh speed and for obtaining an image with high definition has beendifficult to achieve. In the present embodiment, the multi-beam opticalsystem is adopted, and thereby a speed of forming an electrostaticlatent image on a photoconductive drum can be made higher withoutexcessively increasing the number of revolutions of the polygon mirrorand the driving frequency of the LD. In addition, as compared with acase where a plurality of the light sources such as the LDs areallocated at each of different positions, the present embodiment canavoid generation of an alignment error of an allocation position of thelight source and can contribute to improvement of an opticalcharacteristic.

The fθ1 lens 111 and the fθ2 lens 112 have a curvature independentlychanging in two directions, the main-scanning direction and thesub-scanning direction. Here, the fθ1 lens 111 and the fθ2 lens 112correspond to the common (commonly-used) optical element. The powerdistribution of each of the fθ1 lens 111 and the fθ2 lens 112 is set tothe power distribution that applies power to all the light fluxes (allthe light fluxes reflected and deflected by each of the plurality ofreflecting surfaces) Ly to Lk which are reflected and deflected by thepolygon mirror 80 and needed to be guided to each of the plurality ofphotoconductors 401 y to 401 k, so that the light fluxes guided to thephotosensitive surface by the post-deflection optical system A dependingon the incident position of the light fluxes has the predeterminedoptical characteristic (for example, a characteristic that meets apredetermined condition with respect to a beam diameter of the lightflux, a degree of curvature of a scan line, a position of the light fluxon a scanning region, and so on) on the photosensitive surface, with thefθ1 lens 111 and the fθ2 lens 112 working in association with an opticalelement 120 acting on the light beam guided to each of the individualphotoconductors. As described above, the common (commonly-used) opticalelement has a smooth lens surface that acts on all the light fluxesreflected and deflected by each of the plurality of reflecting surfaceson the polygon mirror 80.

As described above, a part of an optical element which has beenindependently provided on each of the conventional photoconductors isintegrated in the common (commonly-used) optical element, and power isapplied to all the light fluxes that need to be guided to the pluralityof photoconductors by the common (commonly-used) optical element.Thereby, the present embodiment can contribute to the reduction in theallocating space of the optical parts in the sub-scanning direction. Inaddition, since the number of items of the optical parts to be allocatedcan be reduced, the present embodiment can avoid deterioration of theoptical characteristic resulting from the alignment error of each of theoptical parts and so on, and contribute to achieving lower cost.

In addition, by integrating a part of the optical element providedindependently on each of the photoconductors in the common(commonly-used) optical element, an inclination angle of each of thereflecting surfaces of the polygon mirror can be set to a small angle,and the allocation space in the sub-scanning direction of the opticalsystem can be made small. In addition, generation of an asymmetricalwave aberration which becomes large when the inclination angle of thereflecting surfaces of the polygon mirror is large can be restricted,and improvement of an image forming characteristic can be achieved.Further, by applying the optical scanner of a configuration as describedabove to the image forming device, the present embodiment can contributeto downsizing of the image forming device and stabilization of imagequality in the image forming processing.

Here, the “predetermined optical characteristic” means an opticalcharacteristic desirable for forming the electrostatic latent image onthe photosensitive surface of the photoconductor. In addition, byproviding a configuration in which the incident light flux on thepolygon mirror from the pre-deflection optical system is focused in thevicinity of the reflecting surfaces (the incident light flux is made tohave a conjugated relationship on the reflecting surfaces of the polygonmirror and on the photosensitive surface of the photoconductor in thesub-scanning direction), shifting of a beam position in the sub-scanningdirection resulting from an inclination of each of the reflectingsurfaces of the polygon mirror is restricted (surface tilt correction).

In addition, power is set to be distributed to the fθ1 lens 111 and thefθ2 lens 112 (common (commonly-used) optical element) in the presentembodiment in a manner that a composite focal point position in thesub-scanning direction on the polygon mirror side is positioned nearerto a side a rotational axis of the polygon mirror 80 is allocated ratherthan the reflecting surfaces 80 y to 80 k of the polygon mirror 80 in anoptical axis direction of the fθ1 lens 111 and the fθ2 lens 112. FIG. 4shows a focal plane FS including the composite focal point position inthe sub-scanning direction on the polygon mirror side of the fθ1 lens111 and the fθ2 lens 112. In FIGS. 4 and 5, a dotted line shows a lightbeam on an incident side of the light flux from the pre-deflectionoptical system (when a scanning angle has a minimum value) and a lenscross section acting on the light beam, a bold line shows the light beamwhen a scanning angle position of scanning light is at a center positionin the main-scanning direction and a lens cross section acting on thelight beam, and a two-dot chain line shows the light beam on a side notapproaching to the pre-deflection optical system in the scanning region(when the scanning angle has a maximum value) and the lens cross sectionacting on the light beam. FIG. 5 enlarges and displays the vicinity ofthe fθ1 lens 111 and the fθ2 lens 112.

A result of optimization indicated that, when the inclination angle ofthe reflecting surfaces of the polygon mirror in the sub-scanningdirection becomes large, the image forming characteristic tends to bedeteriorated. For this reason, in order to obtain a sufficient distancein the sub-scanning direction for separating the optical path by anoptical path separating mirror even with a small inclination angle, afocal point position as described above is set, so that each of thelight fluxes separates from one another in the sub-scanning direction(so that the light flux which has passed the common (commonly-used)optical element advances while space between principal rays widens) evenon a downstream side of the optical path after passing through the fθ1lens 111 and the fθ2 lens 112. By setting the composite focal pointposition of the fθ1 lens 111 and the fθ2 lens 112 in the sub-scanningdirection on the polygon mirror side at the focal plane position asdescribed above, space in which the turning back mirror and so on forseparating each of the light beams are allocated can be secured. FIG. 6is a view showing an example of the power distribution of the fθ1 lens111 in the sub-scanning direction. FIG. 7 is a view showing an exampleof the power distribution of the fθ2 lens 112 in the sub-scanningdirection.

As shown in FIG. 4, each of the optical axis of the cylinder lenses 120y to 120 k is allocated so as to be eccentric to the optical axis sideof a common (commonly-used) optical element 110 more than each of thelight beams with respect to the sub-scanning direction, and has apredetermined inclination. The result of the optimization has indicatedthat the curvature of the scan line on an image surface can be madesufficiently small by providing allocation as described above.

In addition, the pre-deflection optical system 7 shapes light from thelight source 71 into the light flux having a cross-sectional shape witha longer diameter in the main-scanning direction and guides the lightflux toward the polygon mirror 80, and also crosses the light fluxesfrom the plurality of light sources at a position nearer to an upstreamside in an advancing direction of the light fluxes than to thereflecting surfaces 80 y to 80 k of the polygon mirror 80 in thesub-scanning direction.

FIGS. 8 to 11 show a relationship between space between the principalrays (hereinafter referred to as the distance between the principalrays) (vertical axis) of the light flux from the light sources (here, afirst light source and a fourth light source) positioned on both ends inthe sub-scanning direction among the plurality of light sources, and thepositions in the optical axis direction (horizontal axis). In FIGS. 8 to11, a position P shows an exit surface position of the fθ2 lens 112, and“0” in the horizontal axis shows a position of the reflecting surfacesof the polygon mirror 80. Here, FIG. 8 shows a light flux Ly deflectedto a top position in FIG. 4, FIG. 9 shows a light flux Lm deflected to asecond position from the top position in FIG. 4, FIG. 10 shows a lightflux Lc deflected to a third position from the top position in FIG. 4,and FIG. 11 shows a light flux Lk deflected to a bottom position in FIG.4.

As shown in FIGS. 8 to 11, according to the configuration where thelight fluxes from the plurality of light sources are crossed at aposition nearer to the upstream side in the advancing direction of thelight flux than to the reflecting surfaces 80 y to 80 k in thesub-scanning direction, the distance between the principal rays afterpassing through the fθ1 lens 111 and the fθ2 lens 112 gradually changesto be narrower, and a rate of the change of the distance between theprincipal rays with respect to the optical axis direction between thecylinder lens and the image surface can be reduced. The distance betweenthe principal rays after passing through the fθ lenses is reduced to besmall on the cylinder lens in the relationships shown in FIGS. 8 to 11,thereby the light flux on the photosensitive surface can be made to havethe optical characteristic appropriate for the image forming, and afluctuation of a beam pitch on the photosensitive surface can berestricted even in a case where the position of the photosensitivesurface fluctuates in the optical axis direction along with the rotationof the photoconductor drum due to a shape error of the photoconductordrum, the shift (such as inclination and decentering) of a rotationalaxis, abrasion, and so on.

After the light fluxes passed through the fθ1 lens 111 and the fθ2 lens112, space between a plurality of the beams is made to be closer to adesired pitch between beams as the light fluxes advance to thedownstream of the optical path so as not to increase a change amount ofthe pitch between beams with respect to the optical axis direction afterpassing the last cylinder lens to form an image. This is effective forcontrolling by providing a tilt mechanism on the turning back mirror,and for restricting generation of shifting of the pitch between beamswhen a change of a length of the optical path generated by an influenceof a variation of a diameter of the photoconductor drum occurs, in orderto correct tilting of the scan line on the photoconductor or tilting ofa transfer image due to tilting of the photoconductor drum in a case ofa color machine.

Power of the fθ1 lens 111 and the fθ2 lens 112 (common (commonly-used)optical elements) in the sub-scanning direction in a range between atleast the optical axis positions and a position where the principal rayof the light fluxes from the plurality of light sources passes (a rangein the vicinity of a center in the sub-scanning direction) is set to behigher at positions nearer to an outer side than at the center positionin the main-scanning direction (refer to FIGS. 6 and 7).

An effective focal length on the cylinder lens 120 positioned nearer tothe downstream side than the fθ2 lens 112 in the advancing direction ofthe light flux becomes shorter as an incident angle of the light flux inthe main-scanning direction becomes larger. Therefore, in order that thepitch between beams on the image surface of a beam group forming onelatent image is kept to be constant, an angle change amount is made tobe small (position shifting in the sub-scanning direction of the lightflux incident on the incident surface of the cylindrical lens isreduced) by increasing the power in the sub-scanning direction on the fθlens when the incident angle in the main-scanning direction is large(the scanning angle is large), and by lowering a maximum height withrespect to the optical axis at the cylinder lens 120 as much aspossible. In this manner, the constant pitch between beams is finallykept on the image surface.

In addition, the power of the fθ1 lens 111 and the fθ2 lens 112 in thesub-scanning direction in the range between at least the optical axisposition and the position where the principal ray of the light fluxesfrom the plurality of light sources passes (the range in the vicinity ofthe center in the sub-scanning direction) is set to be lower atpositions nearer to the outer side than at the center position in thesub-scanning direction (refer to FIGS. 6 and 7).

The light flux passing through the common (commonly-used) opticalelement has the optical path length which is different depending on atwhich position in the sub-scanning direction the common (commonly-used)optical element passes. According to the present embodiment, a beamdiameter (light focus position) when the light fluxes having a passingposition in the sub-scanning direction different from one another reachan optical element positioned on the downstream side of the optical pathcan be made to be substantially the same, and the variation of theoptical characteristic depending on the passing position in thesub-scanning direction can be restricted. As obvious from FIG. 4, alight beam at outer side has larger inclination in the sub-scanningdirection and longer actual optical path length.

Although a combination of only refractive lenses is provided in thepresent embodiment, a diffractive optical element surface is furtherdesirably added to a refractive lens surface in a manner to restrict amagnification chromatic aberration in the main-scanning direction inorder to restrict a variation of a scan line length due to a wavelengthvariation of a plurality of beams. FIG. 42 is a view showing an examplein which the diffractive optical element surface (indicated by a boldline) is formed on the exit surface side of the fθ2 lens 112 in theconfiguration shown in FIG. 3. FIG. 43 is a view showing an example inwhich the diffractive optical element surface (indicated by a bold line)is formed on the exit surface side of the fθ2 lens 112 in theconfiguration shown in FIG. 32.

FIG. 12 is a view showing an example of allocation of the optical systemof the conventional optical scanner included in a color image formingdevice. As compared with the conventional optical scanner in whichfinite lenses 72J, apertures 73J, and cylinder lenses 74J configuringthe pre-deflection optical system are allocated as many as the number ofthe photoconductor drums shown in FIG. 12, the configuration of theoptical scanner according to the present embodiment shown in FIG. 1 isunderstood to be obviously contributing to space saving and lowering ofcost.

In the present embodiment, there has been shown an example where thecommon (commonly-used) optical element includes two of the fθ lenses.However, the present invention is not limited thereto, and may includethree or more of the lenses. In this manner, the common (commonly-used)optical element is configured with a plurality of the lenses, thereby acurvature of the lens surface of each of the lenses can be set to begentle, processing is facilitated, and the present invention cancontribute to lowering of manufacturing cost and improvement ofprocessing precision, as compared with a case where the common(commonly-used) optical element is configured with one lens.

In the present embodiment, the power distribution is set to becontinuously changing on both the incident surface and the exit surfaceof each of the fθ1 lens and the fθ2 lens constituting the common(commonly-used) optical element. However, the power distribution doesnot need to be set as described above on all the lens surfaces of thecommon (commonly-used) optical element. In general, when the common(commonly-used) optical element is constituted by a plurality of thelenses as described above, the lens positioned on the downstream side inthe light flux advancing direction has larger size in many cases. Thatis, the light flux incident on the lens on the downstream side in theadvancing direction of the light flux has a smaller diameter of beam anda moving distance of the light flux is longer as compared with the lenspositioned on the upstream side in the same scanning angle. Therefore,it is considered that a larger effect is obtained by the powerdistribution continuously changing as described above. For this reason,when the common (commonly-used) optical element as described above isconstituted by the plurality of lenses, the power that changescontinuously as described above is desirably applied on the exit surfaceside of the lens positioned on the most downstream side in the advancingdirection of the light flux (that is, on a side closest to the imagesurface).

Second Embodiment

Next, a second embodiment of the present invention will be described.The present embodiment is a modification example of the first embodimentdescribed above, and in particular, a configuration in the vicinity ofthe fθ lens is different from the first embodiment. Hereinafter, a partidentical to the part which has already been described in the firstembodiment will be attached with the same numerical number, anddescription thereof will be omitted.

FIG. 13 is a plan view showing the optical path in the optical system inthe optical scanner according to the present embodiment. FIG. 14 is aview showing the optical path of the light flux guided to a plurality ofthe photoconductors in the optical scanner according to the presentembodiment in an enlarged manner in the sub-scanning direction in astate where turning back by the mirror is extended. FIG. 15 is anenlarged view of the vicinity of the common (commonly-used) fθ lens inFIG. 14. In FIGS. 14 and 15, a dotted line shows a light beam on anincident side of the light flux from the pre-deflection optical system(when the scanning angle has a minimum value) and a lens cross sectionacting on the light beam, a bold line shows the light beam when thescanning angle position of scanning light is at the center position inthe main-scanning direction and a lens cross section acting on the lightbeam, and a two-dot chain line shows the light beam on a side notapproaching to the pre-deflection optical system in the scanning region(when the scanning angle has a maximum value) and a lens cross sectionacting on the light beam.

In the present embodiment, the fθ1 lens 111 and the fθ2 lens 112 in thefirst embodiment are integrated into an fθ lens 110′ (common(commonly-used) optical element). In this manner, the number of items ofparts of the optical system can be reduced as compared with theconfiguration in the first embodiment, and the present embodiment cancontribute to lowering of cost.

FIGS. 16 to 19 show a relationship between space between the principalrays (hereinafter referred to as the distance between the principalrays) (vertical axis) of the light flux from the light sources (here,the first light source and the fourth light source) positioned on bothends in the sub-scanning direction among the plurality of light sources,and the positions in the optical axis direction (horizontal axis). InFIGS. 16 to 19, a position P shows the exit surface position of the fθlens 110′, and “0” in the horizontal axis shows a position of thereflecting surfaces of the polygon mirror 80. Here, FIG. 16 shows thelight flux Ly deflected to a top position in FIG. 14, FIG. 17 shows thelight flux Lm deflected to a second position from the top position inFIG. 14, FIG. 18 shows the light flux Lc deflected to a third positionfrom the top position in FIG. 14, and FIG. 19 shows the light flux Lkdeflected to a bottom position in FIG. 14.

FIG. 20 shows the power distribution in the sub-scanning direction ofthe fθ lens 110′ according to the present embodiment. FIG. 21 is a viewshowing a position of a light beam incident on the incident surface ofthe cylinder lens 120 y in the sub-scanning direction according to thepresent embodiment. As shown in FIGS. 20 and 21, in a plus side in themain-scanning direction, by increasing the power in the sub-scanningdirection at the fθ lens when the scanning angle of the scan light islarge, a variation of the position in the sub-scanning direction of thelight beam incident on the cylinder lens depending on the scanning angleof the scan light in the main-scanning direction is obviouslyrestricted.

Although a combination of only refractive lenses is provided in thepresent embodiment, a diffractive optical element surface is furtherdesirably added to a refractive lens surface in order to restrict amagnification chromatic aberration in the main-scanning direction,thereby a variation of a scan line length due to a wavelength variationof a plurality of beams is restricted. FIG. 44 is a view showing anexample in which the diffractive optical element surface (indicated by abold line) is formed on the exit surface side of the fθ lens 110′ in theconfiguration shown in FIG. 13. FIG. 45 is a view showing an example inwhich the diffractive optical element surface (indicated by a bold line)is formed on the exit surface side of the fθ lens 110′ in theconfiguration shown in FIG. 24.

Third Embodiment

Next, a third embodiment of the present invention will be described. Thepresent embodiment is a modification example of the second embodimentdescribed above, and in particular, a configuration of the opticalsystem acting on the light flux after passing through the fθ lens isdifferent from the second embodiment. Hereinafter, a part identical tothe part which has already been described in the second embodiment willbe attached with the same numerical number, and description thereof willbe omitted.

Specifically, in the present embodiment, cylinder mirrors 140 y to 140 khaving a concave surface on the incident surface side are adopted inplace of the cylinder lenses 120 y to 120 k in the configuration of thesecond embodiment.

FIG. 22 is a plan view showing a configuration of the optical system ofthe optical scanner according to the present embodiment in a state whereturning back by the mirror is extended. FIG. 23 is a view showing aschematic configuration of an image forming device 900′ including theoptical scanner according to the present embodiment. FIG. 24 is a planview showing the optical path of the optical system of the opticalscanner according to the present embodiment in a state where turningback by the mirror is extended. FIG. 25 is a view showing the opticalpath of the light flux guided by a plurality of the photoconductors inthe optical scanner in an enlarged manner in the sub-scanning directionaccording to the present embodiment in a state where turning back by theturning back mirror is extended. FIG. 26 is an enlarged view of thevicinity of the common (commonly-used) fθ lens in FIG. 25.

FIGS. 27 to 30 show a relationship between space between the principalrays (hereinafter referred to as the distance between the principalrays) (vertical axis) of the light flux from the light sources (here,the first light source and the fourth light source) positioned on bothends in the sub-scanning direction among the plurality of light sources,and the positions in the optical axis direction (horizontal axis). InFIGS. 27 to 30, a position P shows the exit surface position of the fθlens 110′, and “0” in the horizontal axis shows a position of thereflecting surfaces of the polygon mirror 80. Here, FIG. 27 shows thelight flux Ly deflected to a top position in FIG. 26, FIG. 28 shows thelight flux Lm deflected to a second position from the top position inFIG. 26, FIG. 29 shows the light flux Lc deflected to a third positionfrom the top position in FIG. 26, and FIG. 30 shows the light flux Lkdeflected to a bottom position in FIG. 26. In the third embodiment, agraph showing the distance between the principal rays in the light beampassing the center position in the main-scanning direction of the fθlens in the vicinity of the image surface increases, and the distancebetween the principal rays in the light flux passing through positionsof both ends in the main-scanning direction of the fθ lens decreases. Asshown in FIGS. 27 to 30, balance is kept between the cylinder mirror andthe image surface so that the distance between the principal raysincreases as the light beam approaches to the image surface in thecenter part in the main-scanning direction and the distance between theprincipal rays decreases as the light flux approaches the image surfaceon both of the ends, thereby a fluctuation of the distance between theprincipal rays when the image surface position fluctuates is furtherrestricted than the first and the second embodiments. FIG. 31 is a viewshowing the power distribution in the sub-scanning direction of the fθlens 110′ in the present embodiment.

Although a combination of only the refractive lens and the mirror isprovided in the present embodiment, the diffractive optical elementsurface is further desirably added to the refractive lens surface or themirror surface in order to restrict the magnification chromaticaberration in the main-scanning direction, thereby a variation ofpositions in the main-scanning direction due to a variation of awavelength of a plurality of beams is restricted.

Fourth Embodiment

Next, a fourth embodiment of the present invention will be described.The present embodiment is a modification example of the first embodimentdescribed above, and in particular, a configuration of the opticalsystem acting on the light flux after passing through the fθ lens isdifferent from the first embodiment. Hereinafter, a part identical tothe part which has already been described in the first embodiment willbe attached with the same numerical number, and description thereof willbe omitted.

Specifically, in the present embodiment, the cylinder mirrors 140 y to140 k having a concave surface on the incident surface side are adoptedin place of the cylinder lenses 120 y to 120 k in the configuration ofthe first embodiment.

FIG. 32 is a plan view showing a configuration of the optical path inthe optical system of the optical scanner according to the presentembodiment in a state where turning back by the mirror is extended. FIG.33 is a view showing the optical path of the light flux guided by aplurality of the photoconductors in the optical scanner in an enlargedmanner in the sub-scanning direction according to the present embodimentin a state where turning back by the turning back mirror is extended.FIG. 34 is an enlarged view of the vicinity of the common(commonly-used) fθ lens in FIG. 33.

FIGS. 35 to 38 show a relationship between space between the principalrays (hereinafter referred to as the distance between the principalrays) (vertical axis) of the light flux from the light sources (here,first light source and fourth light source) positioned on both ends inthe sub-scanning direction among the plurality of light sources, and thepositions in the optical axis direction (horizontal axis). In FIGS. 35to 38, a position P shows the exit surface position of the fθ2 lens 112,and “0” in the horizontal axis shows a position of the reflectingsurfaces of the polygon mirror 80. Here, FIG. 35 shows the light flux Lydeflected to a top position in FIG. 33, FIG. 36 shows the light flux Lmdeflected to a second position from the top position in FIG. 33, FIG. 37shows the light flux Lc deflected to a third position from the topposition in FIG. 33, and FIG. 38 shows the light flux Lk deflected to abottom position in FIG. 33. In the fourth embodiment, a graph showingthe distance between the principal rays in the light beam passing thecenter position in the main-scanning direction of the fθ lens in thevicinity of the image surface increases, and the distance between theprincipal rays in the light flux passing through positions of both endsin the main-scanning direction of the fθ lens decreases. As shown inFIGS. 35 to 38, balance is kept between the cylinder mirror and theimage surface so that the distance between the principal rays increasesas the light beam approaches to the image surface in the center part inthe main-scanning direction and the distance between the principal raysdecreases as the light flux approaches to the image surface on the bothends, thereby a fluctuation of the distance between the principal rayswhen the image surface position fluctuates is further restricted thanthe first and the second embodiments. FIG. 39 is a view showing thepower distribution in the sub-scanning direction of the fθ1 lens 111 inthe present embodiment. FIG. 40 is a view showing the power distributionin the sub-scanning direction of the fθ2 lens 112 in the presentembodiment.

FIG. 41 is a view showing a result of examining on which lens surface ofthe fθ1 lens and the fθ2 lens in the present embodiment a free-formsurface having the power acting on all the light fluxes guided to aplurality of the photoconductors needs to be formed. In FIG. 41, the topone shows a case in which a curvature of all the lens surfaces arechanged, and others show a case in which the curvature of two lenssurfaces are changed, and are arranged in the order of a smallerevaluation function. The evaluation function is a sum of squares ofvalues obtained by applying a weight on a difference between each of theoptical characteristics and a target optical characteristic. Suchevaluation function is desirably small. From the result shown in FIG.41, in case the common (commonly-used) optical element as shown in thepresent embodiment is adopted, and when the curvatures of two lenssurfaces are changed, a configuration in which the curvature of the lenssurface on the polygon mirror side is changed obviously provides themost preferable optical characteristic.

Although a combination of only the refractive lens and the mirror isprovided in the present embodiment, the diffractive optical elementsurface is further desirably added to the refractive lens surface or themirror surface in order to restrict the magnification chromaticaberration in the main-scanning direction, thereby a variation ofpositions in the main-scanning direction due to a variation of awavelength of a plurality of beams is restricted.

Although the present invention has been described with reference to aspecific embodiment, a variety of changes and modifications can be madewithout departing from the sprit and the scope of the present invention,as obvious to one skilled in the art.

As has been described in detail above, according to the presentinvention, a technique capable of achieving reduction in space ofallocating the optical system and improvement of the opticalcharacteristic of the scan light in the optical scanner that scans thelight flux from the light source on each of the photosensitive surfaceof a plurality of the photoconductors in the main-scanning direction canbe provided.

1. An optical scanner that can scan a light flux from a light source toa photosensitive surface of each of a plurality of photoconductors in amain-scanning direction, comprising: a rotational deflector thatreflects and deflects an incident light flux by a plurality ofreflecting surfaces arranged corresponding to the plurality ofphotoconductors in a rotational direction to scan the incident lightflux in the main-scanning direction, the rotational deflector in whichan inclination angle of the plurality of reflecting surfaces withrespect to a rotational axis of the rotational deflector is set to anangle depending on the photoconductor to which each of the reflectingsurfaces corresponds; a pre-deflection optical system that shapes thelight from the light source into a light flux having a predeterminedcross-sectional shape and introduces the light flux to the rotationaldeflector, and also converges the light flux in a sub-scanning directionin the vicinity of the reflecting surfaces of the rotational deflector;and a post-deflection optical system that introduces the light fluxreflected and deflected by each of the plurality of reflecting surfacesin the rotational deflector to the photosensitive surface of thephotoconductor corresponding to each of the reflecting surfaces, thepost-deflection optical system includes a common (commonly-used) opticalelement that applies power to the light flux reflected and deflected bythe rotational deflector and introduced to each of the plurality ofphotoconductors, so as to make the light flux introduced to thephotosensitive surface by the post-deflection optical system to have apredetermined optical characteristic on the photosensitive surfacedepending on an incident position of the light flux and a plurality ofoptical elements that are arranged on an optical path between the common(commonly-used) optical element and each of the plurality ofphotoconductors corresponding to each of the photoconductors, each ofthe plurality of optical elements having a positive power in asub-scanning direction set depending on the correspondingphotoconductors.
 2. The optical scanner according to claim 1, whereinthe predetermined optical characteristic is at least any of a beamdiameter of the light flux, a degree of curvature of the scan line, anda position of the light flux with respect to a scanning range.
 3. Theoptical scanner according to claim 1, wherein a focal point of thecommon (commonly used) optical element on a side of the rotationaldefector is positioned on a side where the rotational axis of therotational deflector is arranged rather than a side where the reflectingsurfaces of the rotational deflector are arranged in an optical axisdirection of the common (commonly-used) optical element.
 4. The opticalscanner according to claim 1, wherein the power in a range at least froman optical axis position to a position where a principal ray of thelight flux from the light source passes in a sub-scanning direction ofthe common (commonly-used) optical element is set to be larger at anexternal side than at a center position in the main-scanning direction.5. The optical scanner according to claim 1, wherein the power in asub-scanning direction of the common (commonly-used) optical element isset to be weaker at an external side than at a center position in thesub-scanning direction.
 6. The optical scanner according to claim 1,wherein the common (commonly-used) optical element includes a pluralityof lenses arranged in an optical axis direction, and at least any oflens surfaces of at least any lens of the plurality of lenses appliespower to a light flux reflected and deflected by the rotationaldeflector and introduced to each of the plurality of photoconductors, soas to make the light flux introduced to the photosensitive surface bythe post-deflection optical system to have a predetermined opticalcharacteristic on the photosensitive surface depending on an incidentposition of the light flux.
 7. The optical scanner according to claim 1,wherein the pre-deflection optical system shapes the light from aplurality of the light sources into the light fluxes having apredetermined cross-sectional shape and introduces the light fluxes tothe rotational deflector, and also crosses the light fluxes from theplurality of light sources in a sub-scanning direction at a positionnearer to a side of an upstream in an advancing direction of the lightflux than to the reflecting surfaces of the rotational deflector.
 8. Theoptical scanner according to claim 1, wherein the plurality of opticalelements are arranged at positions where the optical elements are madeeccentric to an optical axis side of the common (commonly-used) opticalelement relative to incident light.
 9. An image forming devicecomprising: the optical scanner described in any one of claims 1 to 7and 8; a photoconductor that has an electrostatic latent image formedthereon by a light flux scanned by the optical scanner; and a developingunit that develops the electrostatic latent image formed on thephotoconductor.
 10. An optical scanning method that scans a light fluxfrom a light source to a photosensitive surface of each of a pluralityof photoconductors in a main-scanning direction, comprising: shaping thelight from the light source into a light flux having a predeterminedcross-sectional shape to introduce the light flux to the rotationaldeflector, and also converging the light flux in a sub-scanningdirection in the vicinity of the reflecting surfaces of the rotationaldeflector; reflecting and deflecting an incident light flux by means ofthe rotational deflector which has a plurality of reflecting surfacesarranged corresponding to the plurality of photoconductors in arotational direction, and in which an inclination angle of the pluralityof reflecting surfaces with respect to a rotational axis of therotational deflector is set to an angle depending on the photoconductorto which each of the reflecting surfaces corresponds, to scan theincident light flux in the main-scanning direction; and applying powerto the light flux reflected and deflected by the rotational deflectorand introduced to each of the plurality of photoconductors by means of acommon (commonly-used) optical element so as to make the light fluxintroduced to the photosensitive surface to have a predetermined opticalcharacteristic on the photosensitive surface depending on an incidentposition of the light flux, applying positive power in a sub-scanningdirection set depending on the corresponding photoconductors to thelight flux to which the power is applied by the common (commonly-used)optical element by a plurality of optical elements that are arranged onan optical path between the common (commonly-used) optical element andeach of the plurality of photoconductors corresponding to each of thephotoconductors, and introducing the light flux to which the power isapplied by the plurality of optical elements to the photosensitivesurface of the photoconductor corresponding to each of the reflectingsurfaces.
 11. The optical scanning method according to claim 10, whereinthe predetermined optical characteristic is at least any of a beamdiameter of the light flux, a degree of curvature of the scan line, anda position of the light flux with respect to a scanning range.
 12. Theoptical scanning method according to claim 10, wherein a focal point ofthe common (commonly used) optical element on a side of the rotationaldeflector is positioned on a side where the rotational axis of therotational deflector is arranged rather than a side where the reflectingsurfaces of the rotational deflector are arranged in an optical axisdirection of the common (commonly-used) optical element.
 13. The opticalscanning method according to claim 10, wherein the power in a range atleast from an optical axis position to a position where a principal rayof the light flux from the light source passes in a sub-scanningdirection of the common (commonly-used) optical element is set to belarger at an external side than at a center position in themain-scanning direction.
 14. The optical scanning method according toclaim 10, wherein the power in a sub-scanning direction of the common(commonly-used) optical element is set to be smaller at an external sidethan at a center position in the sub-scanning direction.
 15. The opticalscanning method according to claim 10, wherein the common(commonly-used) optical element includes a plurality of lenses arrangedin an optical axis direction, and at least any of lens surfaces of atleast any lens of the plurality of lenses applies power to a light fluxreflected and deflected by the rotational deflector and introduced toeach of the plurality of photoconductors, so as to make the light fluxintroduced to the photosensitive surface by the post-deflection opticalsystem to have a predetermined optical characteristic on thephotosensitive surface depending on an incident position of the lightflux.
 16. The optical scanning method according to claim 10, wherein thepre-deflection optical system shapes the light from a plurality of thelight sources into the light flux having a predetermined cross-sectionalshape and introduces the light flux to the rotational deflector, andalso crosses the light flux from the plurality of light sources in asub-scanning direction at a position nearer to a side of an upstream inan advancing direction of the light flux than to the reflecting surfacesof the rotational deflector.
 17. The optical scanning method accordingto claim 10, wherein the plurality of optical elements are arranged atpositions where the optical elements are made eccentric to an opticalaxis side of the common (commonly-used) optical element relative toincident light.
 18. The optical scanner according to claim 1, whereinthe post-deflection optical system includes a diffractive opticalelement having power at least in any of the main-scanning direction anda sub-scanning direction.